1. Field of Invention
The present invention relates to electric drive systems. In particular, the invention pertains to pulse width modulation (PWM) strategies employed at an electric drive system.
2. Background Art
Electric machines, in particular electric or hybrid electric vehicles, may employ electrical energy for propulsion via an electric drive system. An electric drive system can encompass a number of components, typically including at least a power circuit and a motor. In this arrangement, the power circuit can controllably transfer power from a power source to the motor to drive a load. Power circuits for electric drive systems designed for electric or hybrid electric vehicles often include inverters for providing three-phase voltage waves from a dc voltage source. Typically an inverter is composed of a plurality of electronic switches that are controllably turned off and on in various combinations to provide desired inverter output voltages.
It is common practice to use pulse width modulation (PWM) techniques to turn the switches on and off. The rate at which the switches are turned on and off is typically determined by motor speed or torque requirements as well as fuel efficiency considerations. Unfortunately, in many cases, switching the various electronic devices on and off at a particular frequency can generate noise audible to the vehicle driver, as well as his passengers. The noise can be distracting and irritating, and is often the subject of consumer complaints.
Various attempts have been made to reduce or eliminate irritating PWM switching noise at an electric vehicle. For example, PWM switching can be performed at a higher frequency, such as 10 kHZ, which is outside the human audible range. However, this solution has its disadvantages. While the higher switching frequency no longer generates noise heard by the operator, it induces power losses in the system that can reduce fuel economy, a long-standing priority for hybrid vehicles, and increase power circuit costs.